The Infection Profile of Humoral Failure | AI Learning Guide

The infections that appear when antibody fails are specific, and the reason is mechanical. Encapsulated bacteria wear a polysaccharide coat that phagocytes cannot grip directly. Antibody has to coat that capsule first, so complement can deposit C3b and the phagocyte can engage the microbe. Remove antibody and the capsule becomes a shield, which is why Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis cause recurrent sinopulmonary and invasive disease. The second clue is enteroviral meningoencephalitis, which appears because neutralizing antibody normally keeps enteroviruses out of the central nervous system. The comparison shows the third point: mucosal IgA and systemic IgG protect different compartments, so losing IgA alone gives mucosal disease, while losing systemic IgG gives invasive infection.

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The signature of humoral failure

Recurrent sinopulmonary infection with encapsulated bacteria, plus enteroviral meningoencephalitis, is the characteristic profile of antibody deficiency. It differs from the innate patterns covered earlier because the missing step is antibody-mediated opsonization and neutralization, not complement or phagocyte killing itself.

Why encapsulated bacteria dominate

The polysaccharide capsule blocks direct phagocyte recognition. Antibody must coat the capsule before complement can deposit C3b and before phagocytes can engage the microbe through Fc receptors. Remove antibody and the capsule becomes an effective shield, so these organisms cause invasive disease.

Mucosal versus systemic antibody protection

Secretory IgA (mucosal)

Produced locally at mucosal surfaces
Blocks attachment and entry of pathogens
Loss causes recurrent mucosal infection, rarely invasive disease

Systemic IgG (blood and tissue)

Circulates in blood and interstitial fluid
Opsonizes encapsulated bacteria and neutralizes viruses
Loss causes invasive infection and enteroviral meningoencephalitis